An exhaust-cleaning system and a process are known from the articles “Abgasnachbe-handlungssystem zur Erfüllung von Euro IV Grenzwerten bei PKW-Dieselmotoren” [Exhaust aftertreatment system for complying with Euro IV limit values in passenger automobile diesel engines] by Aust, Tost, Wiβler, Fischer and Zürbig, and NOx-Verminderung für Nutzfahr-zeugmotoren mit Harnstoff-SCR-Kompaktsystemen [Lowering the levels of NOx for commercial vehicle engines using urea-SCR compact systems] (Gesteuerter Diesel-Katalysator, GD-KAT) by Jacob, Emmerling, Döring, Graf, Harris, van den Tillaart and Hupfeld from VDI Fortschrittsberichte, Vol. 12, No. 348, 1998. In this case, the reagent is admixed with the exhaust gas downstream of the oxidation catalytic converter and upstream of the reduction catalytic converter, as seen in the direction of flow of the exhaust gas. In addition to the function of removing soot particles and unburnt hydrocarbons and carbon monoxide which are present in the exhaust gas by oxidation, the oxidation catalytic converter also has the function of shifting the ratio of the nitrogen oxides NO and NO2 which are present in the exhaust gas toward NO2. Therefore, at the oxidation catalytic converter nitrogen monoxide NO is partially oxidized to form nitrogen dioxide NO2.
The increased nitrogen dioxide content which is present in the exhaust gas after it has flowed through the oxidation catalytic converter accelerates the catalytic reaction at the reduction catalytic converter and thereby leads to higher deNOxing rates. The catalytic reaction at the reduction catalytic converter is what is known as selective catalytic reduction (SCR), according to which nitrogen oxides which are present in the exhaust gas, i.e. both nitrogen monoxide and nitrogen dioxide, are reacted to form nitrogen and water by consuming a reducing agent and in the presence of oxygen. The acceleration of the catalytic reaction at a SCR catalytic converter by increasing the relative NO2 content is known from DE 28 32 002 B2. Ammonia or hydrocarbons are known as reducing agents for selective catalytic reduction. Ammonia itself, aqueous ammonia or substances which release ammonia, such as urea or ammonium carbamate, in solid or liquid form, are known as reagents for introducing ammonia into the exhaust gas.
The upstream connection of an oxidation catalytic converter is advantageous if the temperature of the exhaust gas from time to time is below 250° C. This is because at such a low temperature increasing the nitrogen dioxide content by means of the oxidation catalytic converter has an extremely advantageous effect on the catalytic conversion at the reduction catalytic converter.
However, a drawback is that the rate at which nitrogen dioxide NO2 is formed from nitrogen monoxide NO at the oxidation catalytic converter decreases considerably at temperatures below 200° C., so that at temperatures below 200° C. only insufficient deNOxing can be achieved even with a combination of oxidation catalytic converter and reduction catalytic converter.